When a conversion is aborted, the contents of the data registers, ADC_RH and ADC_RL,
are not altered. However, they continue to be the values transferred after the completion
of the last successful conversion. If the conversion was aborted by a reset, ADC_RH and
ADC_RL return to their reset states.
15.4.4.4 Power control
The ADC module remains in its idle state until a conversion is initiated. If ADACK is
selected as the conversion clock source, the ADACK clock generator is also enabled.
Power consumption when active can be reduced by setting ADC_SC3[ADLPC]. This
results in a lower maximum value for f
ADCK
(see the data sheet).
15.4.4.5 Sample time and total conversion time
The total conversion time depends on the sample time (as determined by
ADC_SC3[ADLSMP]), the MCU bus frequency, the conversion mode (8-bit, 10-bit or
12-bit), and the frequency of the conversion clock (f
ADCK
). After the module becomes
active, sampling of the input begins.ADC_SC3[ADLSMP] selects between short (3.5
ADCK cycles) and long (23.5 ADCK cycles) sample times. When sampling is complete,
the converter is isolated from the input channel and a successive approximation algorithm
is performed to determine the digital value of the analog signal. The result of the
conversion is transferred to ADC_RH and ADC_RL upon completion of the conversion
algorithm.
If the bus frequency is less than the f
ADCK
frequency, precise sample time for continuous
conversions cannot be guaranteed when short sample is enabled (ADC_SC3[ADLSMP]
= 0). If the bus frequency is less than 1/11th of the f
ADCK
frequency, precise sample time
for continuous conversions cannot be guaranteed when long sample is enabled
(ADC_SC3[ADLSMP] = 1).
The maximum total conversion time for different conditions is summarized in the table
below.
Table 15-2. Total conversion time vs. control conditions
Conversion type
ADICLK
ADLSMP
Max total conversion time
Single or first continuous 8-bit
0x, 10
0
20 ADCK 5 bus clock cycles
Single or first continuous 10-bit or 12-bit
0x, 10
0
23 ADCK 5 bus clock cycles
Single or first continuous 8-bit
0x, 10
1
40 ADCK 5 bus clock cycles
Single or first continuous 10-bit or 12-bit
0x, 10
1
43 ADCK 5 bus clock cycles
Single or first continuous 8-bit
11
0
5 µs + 20 ADCK + 5 bus clock cycles
Table continues on the next page...
Chapter 15 Analog-to-digital converter (ADC)
MC9S08PA4 Reference Manual, Rev. 5, 08/2017
NXP Semiconductors
311
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